1. Field of the Invention
This invention relates generally to laser assemblies, and more particularly to a widely tunable laser assembly with an integrated optical modulator.
2. Description of the Related Art
(Note: This application references a number of different patents and/or publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different patents and/or publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these patents and/or publications is incorporated by reference herein.)
A compact, high-performance widely-tunable integrated laser/modulator chip would be a key component of a tunable transmitter that can dramatically lower the barriers to deployment and operation of high capacity, dense-wavelength division-multiplexing (DWDM) networks. Traditional non-tunable implementations of DWDM transmitters have discouraged the integration of laser source and modulator because of the high cost of these individual components combined with the fact that separate part numbers for each wavelength and its spare would have to be inventoried. Several implementations of such co-packaged transmitters exist for 10 Gb/s transmission in which the laser and modulator are fabricated on separate chips and coupled together by micro-optics. Given systems employing as many as 100 or more wavelengths, this model has contributed to the mountains of inventory associated with the current telecom build-out. However, a widely-tunable laser with full band coverage would resolve this problem by using a single part for all channels with minimal spares, and would give an economic impetus for the further integration of source and modulator into one hermetic package.
The present invention describes an approach wherein a laser and modulator are fabricated by monolithic integration on a single indium phosphide (InP) chip. The laser is a widely-tunable Sampled Grating Distributed Bragg Reflector (SG-DBR) laser that is made possible by an InP-based technology platform that can integrate active waveguide, passive waveguide, and grating reflector sections, all of which can be tuned by current injection.
The modulator is a Mach-Zehnder (MZ) modulator, which is the structure of choice for long-reach transmission systems of 10 Gb/s or more because of its favorable chirp and extinction characteristics. The MZ modulator includes two curved waveguides whose relative optical phase length can be adjusted at high speed with a modulation voltage through the electro-optic effect, and two multimode interference (MMI) couplers that successively split the incoming light into two paths and then constructively or destructively combine the light on the output depending on the modulated phase difference. As a discrete component, the MZ modulator is typically fabricated on lithium niobate (LiNbO3) or gallium arsenide (GaAs) substrates with device lengths of several centimeters, thus requiring the use of a traveling-wave electrode geometry to overcome capacitance limitations.
A monolithically integrated laser and modulator presents a number of opportunities. Lower voltage and smaller modulator size through the use of the quadratic electro-optic effect in InP allow for a compact chip (4×0.5 mm2) and package (30×10 mm2), as well as lower power dissipation in the modulator. Low coupling loss between laser and modulator calls for reduced laser launch power and hence lower power dissipation in the laser.
Additional benefits of the InP integration platform include modulator chirp control through tuning current injection, additional amplification stages for higher power output, as well as integrated tap photodiodes for modulator bias control. Furthermore, the developed technology can be used to supply enabling building blocks to provide additional functionality including alternative data encoding formats and modulation techniques which will become necessary for next generation systems due to the combination of high bit rates and small channel spacing.
Several embodiments of InP MZ modulators with and without integrated lasers have been disclosed in the literature [1,2,3]. For example, the prior art has disclosed a tunable laser with an integrated MZ modulator [1,2]; however, the tuning range was limited by the laser design to the amount of index shift achievable in InP materials and in practice to <2.5 nm. Additional prior art has been disclosed on the integration of widely tunable lasers with electro-absorptive modulators [4,14]; however, this structure has limited dispersion tolerance due to positive chirp inherent in the bulk Franz-Keldysh modulator used for operation over the wide wavelength tuning range of the laser. Other art has integrated multiple smaller tuning range lasers with a single modulator to cover a wider wavelength range [21]; however, this approach suffers an inherent loss due to the need to couple the multiple lasers into single input, and the additional issues relating to temperature change in the modulator when tuning the individual lasers to the desired frequency.
The present invention improves upon the prior art by integrating a single laser where the tuning range is larger than what is achievable through index change (in practice >40 nm) with an interferometric modulator whose chirp can be optimized and controlled over such a wide wavelength range.
Conventional InP MZ modulators suffer from additional attenuation when voltage is applied for the necessary phase shift. This problem degrades the extinction ratio and prevents negative chirp in conventional InP modulators. The prior art has disclosed inserting a π (i.e. 180°) phase shift between the arms and changing the splitting ratio of the input and output splitters in the MZ modulator to allow for simultaneous high extinction and negative chirp [5,6,7,17]. Additional prior art has disclosed the use of additional voltage electrodes to change the value of this phase shift after fabrication [8,17]. These approaches in the prior art have deficiencies in that the range of differential phase shift between the arms (without any bias applied) must be tightly controlled in the device and deviations in fabrication, over temperature and over life need to be compensated with voltage, inducing additional undesirable loss and extinction ratio changes.
The present invention improves upon the prior art by using electrodes that inject current to adjust the phase shift between the arms to any value that is desired. The present invention allows 10× less loss for a given phase shift allowing for a larger range of phase shifts to be achieved without degrading the extinction ratio due to loss imbalance. This improvement creates a MZ modulator that has characteristics much more similar to LiNbO3 or GaAs modulators in that the devices can be operated with any built-in differential phase shift between the arms, and not necessarily 180 degrees as stated in the prior art [5,17].
One of the serious issues in the prior art with integrating a laser monolithically with a MZ modulator is that the modulator designs shown in the prior art reflect light differently between the on and off state of the modulator [9,10]. This reflection slightly perturbs the lasing wavelength of the on-chip laser imparting additional chirp on the modulated light signal and degrading fiber optic transmission.
The present invention overcomes this limitation by using a 2×2 multimode interference (MMI) coupler acting as a combiner in the output of the MZ modulator. This improvement causes the on and off state of the modulator to have the same reflectivity which does not impart any frequency chirp due to the laser.